Not waterproof, but I just got a simple leather ‘wallet’ for my S4 that holds a few cards and some notes. The leather’s done a good job of protecting the phone, droped from moving bike with no problems!
This kind of thing (although I’m sure it was nearer £15):
http://www.ebay.co.uk/itm/WALLET-LEATHER-FLIP-CASE-COVER-FOR-SAMSUNG-GALAXY-S4-I9500-FREE-SCREEN-PROTECTOR-/181256816817#ht_2224wt_959
Positives, £500 phone doesn’t look like a childs toy and the case folds on the back of the phone to form a stand for watching films on the train etc.
Negatives, the S4 uses a magnet in the case to sense when it’s closed and turn off the screen, but it senses it when you fold it back the other way to use it as a phone. No idea how iPhones know when to switch off, if it’s the same it’s irritating.